Solar Power Management

TT30 has three 200 W(peak)
Voltech solar glass panels on the roof (Vmp=18.98 V, Imp=10.54 A, 1580 x 810 x 35 mm,
14.6 kg each). They are connected through a shunt
(to measure their output) to a
Votronic MPP 420 Duo Digital Li Solar Controller, capable of managing 420 W(peak) input. This is an
MPPT controller with primary output to a variety of battery types including AGM and LiFePO4.
It has a secondary output to trickle charge the starter battery. Evidently the TT philosophy is that
420 W(peak) is adequate to maintain the camper battery even though three panels are capable of much
more, but only some of the time — it is more important to have enhanced charging at low sun angles
and in low light conditions and this is where the MPPT feature and extra panel will give value.

Note that I do not use the Votronic Li program for the installed LiFePO4 battery. In my
opinion the U1 charging voltage limit of 15.2 V is too high, and the handover to U2 float voltage of
13.7 V is unspecified. Instead, I use the GEL program which, at 20oC, has U1 voltage of 14.3 V
(3.575 V per cell) and U2 voltage of 13.8 V (3.45 V per cell).

I have disconnected the supplied battery temperature sensor to ensure these voltages are not exceeded
at lower temperatures. In fact, I use this input to the Solar Controller to reduce the U1 voltage limit
further, to 13.8 V (3.45 V per cell), by simulating a temperature input of ~47°C with a resistor as follows:

I have measured the characteristics of the remote temperature sensor — it is a PTC thermistor in a
bolt-on metal jacket. I immersed the sensor in water at different temperatures and obtained the graph here.

Note in particular that at the reference temperature of 20°C the thermistor resistance is 1.88 kOhm. At
this temperature the controller has setpoints of 14.3 V for Gel (U1) and 13.8 V for Float (U2). To cause the
Gel U1 voltage to be 13.8 V the graph shows the resistance of the sensor should be 2.3 kOhm (47°C). The
Float or U2 voltage will then be 13.4 V.

Courtesy of TravelTrucks, with minor modifications by me, this is the circuit diagram for the solar
system:

The manual for the Votronic MPP 420 Duo Digital Li Solar Controller
is here for reference.

A Votronic Solar Computer is connected to the Solar Controller by an 8-core flat cable with RJ45 plugs.
The Computer gives readouts of the V, A, W, and cumulative Ah and Wh from the panels. It is a rudimentary
display, offering little useful additional information. The manual for the Votronic Solar Computer
is here for reference.